According to a microphone including a microphone chip and a circuit element, the microphone chip and the circuit element are connected by a high-impedance wiring. However, in a case that a high-impedance wiring is used, noise is likely to be picked up by the wiring. When noise is picked up by the wiring, the noise is added to the signal transmitted from the microphone chip to the circuit element, and the signal-noise (S/N) ratio of a detection signal becomes poorer. Thus, the microphone is desirably structured such that noise is not picked up by a wiring portion connecting the microphone chip and the circuit element as much as possible.
A microphone accommodating a microphone chip and a circuit element inside a package is described in Japanese Unexamined Patent Publication No. 2006-211468 (Patent Document 1). FIG. 1 is a cross-sectional diagram of a microphone 11 (a package module) described in Patent Document 1. With this microphone 11, a microphone chip 12 and a circuit element 13 are bonded next to each other on the upper surface of a printed wiring board 14, and a cover 15 placed on the upper surface of the printed wiring board 14 covers the microphone chip 12 and the circuit element 13. The microphone chip 12 is connected to the printed wiring board 14 by a solder ball 16 provided on the lower surface and a penetrating electrode 17 penetrating through the top and bottom of the microphone chip 12. Also, the circuit element 13 is connected to the printed wiring board 14 by a solder ball 18 provided on the lower surface. Moreover, the microphone chip 12 and the circuit element 13 are connected by a pattern wiring provided to the printed wiring board 14.
According to the structure of the microphone 11 of Patent Document 1, the pattern wiring connecting the microphone chip 12 and the circuit element 13 is provided on the upper surface of the printed wiring board 14, and is surrounded by a package formed from the printed wiring board 14 and the cover 15. Thus, if a ground conductor is provided to the entire printed wiring board 14 and the cover 15, the influence of external noise is not likely to be received.
Furthermore, according to a microphone 21 shown in FIG. 2, a microphone chip 12 and a circuit element 13 are mounted, next to each other, on the lower surface of a cover 15, and a printed wiring board 14 is joined below the cover 15. With this microphone 21, the microphone chip 12 and the circuit element 13 are connected by a high-impedance bonding wire 22. The circuit element 13 is connected to a bonding pad 24 provided on the lower surface of the cover 15 by a low-impedance bonding wire 23, and the bonding pad 24 is joined to a connection pad 25 of the printed wiring board 14 by conductive resin 26. A hatched layer in FIG. 2 is a ground conductor 27 connected to the ground. Such a microphone may be a microphone described in Japanese Patent Application No. 2010-52643 as filed by the applicant, for example).
With the microphone 21 having the structure as shown in FIG. 2, the microphone chip 12 and the circuit element 13 are directly connected by the bonding wire 22. Accordingly, the high-impedance bonding wire 22 which easily picks up noise is surrounded by the ground conductor 27 of the package, and is not particularly affected by the noise from outside. Also, the bonding wire 23 for connecting the circuit element 13 to the printed wiring board 14 is partially positioned near the surface of the package, and is not covered by the ground conductor 27 near the joined portion of the bonding pad 24 and the connection pad 25 with respect to some directions. That is, noise may come from upper oblique side of the microphone 21 to the bonding wire 23. However, since the impedance of the bonding wire 23 is low, it does not easily pick up noise, and noise does not particularly become an issue.
The applicant filed Japanese Patent Application No. 2010-125527 for a microphone whereby a microphone chip and a circuit element are vertically placed to reduce the mounting area at the time of mounting of the microphone to a circuit board or the like by mounting one of the microphone chip and the circuit element on the substrate and the other on a cover. FIG. 3 shows an example of such a microphone.
According to a microphone 31 shown in FIG. 3, a microphone chip 12 is mounted inside a concave portion of a cover 15, and a bonding pad 24 provided on the lower surface of the cover 15 and the microphone chip 12 are connected by a bonding wire 22. Also, a circuit element 13 is mounted on the upper surface of the printed wiring board 14, and a connection pad 25 provided on the upper surface of the printed wiring board 14 and the circuit element 13 are connected by a pattern wiring provided on the upper surface of the printed wiring board 14. The cover 15 is fixed on the upper surface of the printed wiring board 14 by joining the bonding pad 24, the connection pad 25 and the like by conductive resin 26. As a result, the microphone chip 12 and the circuit element 13 are electrically connected via the bonding wire 22, the bonding pad 24, the conductive resin 26, the connection pad 25, and the pattern wiring of the printed wiring board 14.
In the case of the microphone 31 in FIG. 3, the bonding wire 22, which is a part of the wiring connecting the microphone chip 12 to the circuit element 13, is partially positioned near the surface of the package, and is not covered by a ground conductor 27 (hatched in FIG. 3) near the joined portion of the bonding pad 24 and the connection pad 25 with respect to some directions. That is, if, as shown in FIG. 3, noise N comes from the diagonally upward side of the microphone 31 to the bonding wire 22, the bonding pad 24 possibly picks up the noise by the bonding wire 22. Moreover, since the bonding wire 22 is a high-impedance wiring, such a microphone 31 has a structure susceptible to external noise.
Additionally, to improve the noise immunity of such a microphone 31, at least the lower portion of the outer peripheral surface of the cover 15 is covered by a conductive layer, and the conductive layer is connected to the ground. However, in the fabrication process of the microphone 31, a large number of microphones are integrally fabricated at the same time, and separate microphones are obtained by cutting in the final step, and the outer peripheral surface of the microphone 31 is the cutting surface in the fabrication process. Thus, if a conductive layer is to be formed on the outer peripheral surface of the microphone 31, the conductive layer has to be added separately to each microphone 31 which has been cut, and there is a problem that the production yield of the microphone 31 is reduced.
As described above, external noise is not particularly an issue in the case a microphone chip and a circuit element are mounted, next to each other, on a substrate or a cover. However, as shown in FIG. 3, with a microphone where a microphone chip and a circuit element are vertically placed by separately mounting the microphone chip and the circuit element on the substrate side and the cover side and joining the cover on the substrate, the influence of external noise is great. Particularly, the influence of external noise is great with a structure in which the microphone chip and the circuit element are electrically connected via pad portions provided to the cover and the substrate. Such an inconvenience is present not only for a microphone, but also for a semiconductor device having a similar assembly structure.
In view of the technical problem described above, it is desired to reduce the influence of external noise and to improve the noise immunity of a semiconductor device according to which a semiconductor element such as a microphone chip and the like and a circuit element are vertically placed.